The present invention relates to the compression of digital data, and more particularly to a method and apparatus for communicating compressed, motion compensated digitized video signals.
Television signals are conventionally transmitted in analog form according to various standards adopted by particular countries. For example, the U.S. has adopted the standards of the National Television System Committee ("NTSC"). Most European countries have adopted either PAL (Phase Alternating Line) or SECAM standards.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
A substantial amount of digital data must be transmitted in any digital television system. This is particularly true where high definition television ("HDTV") is provided In a digital television system, a subscriber receives the digital data stream via a receiver/descrambler that provides video, audio, and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must be transmitted.
The video portion of a television signal comprises a sequence of video images (typically "frames") that together provide a moving picture. In digital television systems, each line of a video frame is defined by a sequence of digital data samples referred to as "pixels". A large amount of data is required to define each video frame of a television signal. For example, 7.4 megabits of data is required to provide one video frame at NTSC resolution. This assumes a 640 pixel by 480 line display is used with 8 bits of intensity value for each of the primary colors red, green, and blue. High definition television requires substantially more data to provide each video frame. In order to manage this amount of data, particularly for HDTV applications, the data must be compressed.
Video compression techniques enable the efficient transmission of digital video signals over conventional communication channels. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the important information in a video signal. The most powerful compression systems not only take advantage of spatial correlation, but can also utilize similarities among adjacent frames to further compact the data.
Motion compensation is one of the most effective tools for accounting for and reducing the amount of temporal redundancy in sequential video frames. One of the most effective ways to apply motion compensation in video compression applications is by differential encoding. In this case, the differences between two consecutive images (e.g., "frames") are attributed to simple movements. A signal encoder includes a motion estimator that estimates or quantifies these movements by observing the two frames, and provides motion vector data for transmission along with the compressed video data to a receiver. The transmitted video data comprises the differences between a current frame and prior prediction frame. The receiver includes a corresponding decoder that uses the received information to transform the previous frame, which is known, in such a way that it can be used to effectively predict the appearance of the current frame, which is unknown.
In this way, the amount of information needed to represent the image sequence can be significantly reduced, particularly when the motion estimation model closely resembles the frame to frame changes that actually occur. This technique can result in a significant reduction in the amount of data that needs to be transmitted once simple coding algorithms are applied to the prediction error signal. An example of such a motion compensated video compression system is described by Ericsson in "Fixed and Adaptive Predictors for Hybrid Predictive/Transform Coding", IEEE Transactions on Communications, Vol. COM-33, No. 12, Dec. 1985.
A problem with differential encoding is that it is impossible to ensure that the prediction signals derived independently at the encoder and decoder sites are identical at all times. Differences can arise as a result of transmission errors or whenever one of the two units is initialized. Thus, for example, a television channel change will render the prior frame data meaningless with respect to a first frame of a new program signal To deal with this problem, it is necessary to provide some means of periodic refreshing.
One method of refreshing the image is to periodically switch from differential encoding ("DPCM") to nondifferential encoding ("PCM"). For example, in a thirty frame per second system, the screen could be completely refreshed at one second intervals by inserting a PCM frame after every twenty-nine DPCM frames In this way, channel acquisition and the correction of transmission errors could be guaranteed after a delay of no more than one second. It is assumed here that the switch to PCM coding can be done without affecting the perceived quality of the reconstructed video. However, this is only possible in a variable bit rate encoding system using rate buffers to control fluctuations in the input and output data rates. Such a system is described by Chen and Pratt, in "Scene Adaptive Coder", IEEE Transactions on Communications, Vol. COM-32, No. 3, March 1984. Unfortunately, the resulting large number of bits due to the less efficient PCM encoding is difficult for the encoder and decoder buffers to handle, and measures used to control it may cause visible artifacts to appear in the reconstructed image.
To overcome this problem, segments or blocks of the image can be refreshed on a distributed basis. By assigning a different counter to each segment and systematically or randomly setting the initial count for each one, it is possible to attain the same refresh interval while maintaining a constant distribution of bits. It is even possible to eliminate the counters and instead, randomly refresh each segment based on a suitable probability distribution.
However, the motion compensation process itself introduces a new problem. The motion estimator does not limit the block displacements in such a way as to prevent overlap between refreshed and nonrefreshed regions of the image. For example, if one region of the image is refreshed during the transmission of a given frame, then there will exist an adjacent region in the same frame that has not yet been refreshed but is due to be refreshed during the next frame interval. Obviously, this unrefreshed region is much more likely to contain at least one error. If this less reliable data in the unrefreshed region is used to predict the appearance of certain segments of the next frame, then those segments of that frame will also be subject to errors. It is therefore possible that a recently refreshed region will cease to be accurate after only one frame. In a motion compensated system, this result tends to occur whenever there is movement from an unrefreshed region to a refreshed region, causing a recently refreshed segment of the image to immediately diverge from the corresponding encoder segment, even though no transmission errors occur after refreshing. Once again, the acquisition time and the duration of artifacts due to transmission errors can become unbounded.
A method for refreshing motion compensated sequential video frames that does not suffer from the above-mentioned problems is disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 07/614,939, filed Nov. 16, 1990, and entitled "Method and Apparatus for Refreshing Motion Compensated Sequential Video Images", incorporated herein by reference. In the refresh technique disclosed therein, video images are divided into a plurality of adjacent regions. The image area is refreshed during a refresh cycle by communicating a different region in each successive video image without motion compensation. An image area defined by the aggregate of the regions is progressively refreshed by the nonmotion compensated processing during the refresh cycle. The motion compensation process is controlled to prevent data contained in regions not yet refreshed during a current refresh cycle from corrupting data contained in regions that have been refreshed during the current refresh cycle.
In order to implement a cost-efficient HDTV system, it would be advantageous to process the video data using multiple encoders operating in parallel. Such a scheme would enable the use of low speed encoders and decoders to process video sequences which contain a large number of pixels, such as HDTV signals. The present invention provides a method and apparatus for implementing a high definition television system that provides both motion compensation and refreshing using multiple low speed processors. Complete independence among the processors is maintained without significantly compromising system performance.